In automation technology, the most varied of field devices are applied for determining and/or monitoring at least one process variable, especially a physical or chemical, process variable. Involved, for example, are fill-level measuring devices, flow measuring devices, pressure and temperature measuring devices, pH-redox potential measuring devices, conductivity measuring devices, etc., which register the corresponding process variables, fill level, flow, pressure, temperature, pH-value, and conductivity, etc. The associated measuring principles are known from a large number of publications.
A field device typically includes at least one sensor unit coming at least partially and at least at times in contact with the process, and an electronics unit, which serves, for example, for signal registration, evaluation and/or feeding. Referred to as field devices in the case of the present invention are, in principle, all measuring devices, which are applied near to the process and which deliver, or process, process relevant information, thus, also remote I/Os, radio adapters, and, generally, electronic components, which are arranged at the field level. A large number of such field devices are manufactured and sold by the Applicant.
In a number of corresponding field devices, electromechanical transducer units are used. An example of this are vibronic sensors, such as, for example, vibronic fill level or flow measuring devices. They are also used in ultrasonic, fill-level measuring devices or-flow measuring devices. To visit each type of field device having an electromechanical transducer unit and to explore underlying principles of the different types separately and in detail would be superfluous. Thus, for purposes of simplicity, where reference is taken to particular field devices, the following description is limited, by way of example, to fill-level measuring devices with an oscillatable unit.
The oscillatable unit of such fill-level measuring devices, also as referred to vibronic sensors, is, for example, an oscillatory fork, a single tine or a membrane. The oscillatable unit is excited during operation by means of a driving/receiving unit, usually in the form of an electromechanical transducer unit, to execute mechanical oscillations. The electromechanical transducer unit can be, for example, a piezoelectric, electromagnetic or even magnetostrictive driving/receiving unit. Corresponding field devices are manufactured by the Applicant in great variety and are sold, for example, under the marks, LIQUIPHANT and SOLIPHANT. The underpinning measuring principles are basically known. The driving/receiving unit excites the mechanically oscillatable unit by means of an electrical exciting signal to execute mechanical oscillations. Conversely, the driving/receiving unit can receive the mechanical oscillations of the mechanically oscillatable unit and convert them into an electrical, received signal. The driving/receiving unit can be either a separate driving unit and a separate receiving unit, or a combined driving/receiving unit.
For exciting the mechanically oscillatable unit, the most varied of methods, both analog as well as also digital, have been developed. In many cases, the driving/receiving unit is part of a fed back, electrical, oscillatory circuit, by means of which the exciting of the mechanically oscillatable unit to execute mechanical oscillations occurs. For example, for a resonant oscillation, the oscillatory circuit condition, according to which the amplification factor is ≥1 as well as all phases arising in the oscillatory circuit must sum to a multiple of 360°, must be fulfilled. This has the result that a certain phase shift between the exciter signal and the received signal must be assured. For this, the most varied of solutions are known. In principle, the setting of the phase shift can be performed, for example, by application of a suitable filter, or even be controlled by means of a control loop to a predeterminable phase shift, the desired value. Known from DE102006034105A1, for example, is to use a tunable phase shifter. The additional integration of an amplifier with a tunable amplification factor for additional control of the oscillation amplitude was described, in contrast, in DE102007013557A1. DE102005015547A1 provides the application of an all-pass filter. The setting of the phase shift is, moreover, possible by means of a method involving frequency sweep, such as disclosed, for example, in DE102009026685A1, DE102009028022A1, and DE102010030982A1. The phase shift can, however, also be controlled by means of a phase control loop (phase-locked-loop, PLL) to a predeterminable value. Such an excitation method is subject matter of DE102010030982A′1.
Both the exciter signal as well as also the received signal are characterized by frequency, amplitude and/or phase. Changes in these variables are then usually taken into consideration for determining the particular process variable, such as, for example, a predetermined fill level of a medium in a container, or even the density and/or viscosity of a medium. In the case of a vibronic limit level switch for liquids, for example, of interest is whether the oscillatable unit is covered by the liquid or freely oscillating. These two states, the free state and the covered state, are, in such case, distinguished, for example, based on different resonance frequencies, thus, a frequency shift, or based on damping of the oscillation amplitude.
The density and/or viscosity can, in turn, only be ascertained with such a measuring device, when the oscillatable unit is covered by the medium. Known from DE10050299A1, DE102006033819A1 and DE102007043811A1 is to determine the viscosity of a medium based on the frequency-phase curve (ϕ=g(f)). This procedure is based on the dependence of the damping of the oscillatable unit by the viscosity of the medium. In order to eliminate the influence of the density on the measuring, the viscosity is determined based on a frequency change caused by two different values for the phase, thus, by means of a relative measurement. For determining and/or monitoring the density of a medium, in contrast, according to DE10057974A1, the influence of at least one disturbing variable, for example, the viscosity, on the oscillation frequency of the mechanically oscillatable unit is ascertained and compensated. In DE102006033819A1, it is, furthermore, taught to set a predeterminable phase shift between the exciter signal and the received signal, in the case of which effects of changes of the viscosity of the medium on the mechanical oscillations of the mechanically oscillatable unit are negligible. At this phase shift, an empirical formula for determining the density can be created.
The driving/receiving unit is, as already mentioned, as a rule, embodied as an electromechanical transducer unit. Often, it includes at least one piezoelectric element in the most varied of embodiments. By using the piezoelectric effect, a high efficiency can be achieved. In such case, efficiency is with reference to the efficiency of changing electrical into mechanical energy. Corresponding piezoceramic materials based on LZT (lead zirconium titanate) are, normally, suitable for use at temperatures up to 300° C. There are piezoceramic materials, which keep their piezoelectric properties at temperatures above 300° C.; these have, however, the disadvantage that they are significantly less effective than the materials based on LZT. For use in vibronic sensors, these high temperature materials are, moreover, only conditionally suitable, due to the large differences in the coefficients of thermal expansion of metals and ceramic materials. Because of their function as force providers, the at least one piezoelectric element must be connected with a membrane (which is part of the oscillatable unit) by a force transmitting connection. Especially in the case of high temperatures, however, quite often, large mechanical stresses arise, which can lead to a breaking of the piezoelectric element and, associated therewith, a total failure of the sensor.
An alternative, which can be better suited for use at high temperatures, is provided by so-called electromagnetic driving/receiving units, such as, for example, described in the documents, WO 2007/113011 and WO 2007/114950 A1. The changing of electrical energy into mechanical energy occurs, in such case, via a magnetic field. A corresponding electromechanical transducer unit includes at least one coil and a permanent magnet. By means of the coil, an alternating magnetic field passing through the magnet is produced, and, via the magnet, a periodic force is transferred to the oscillatable unit. Usually, the transfer of this periodic force occurs similarly to the principle of the solenoid, which sits centrally on the membrane. In this way, the driving/receiving unit is applicable for a temperature range between −200° C. and 500° C. Often, there is, however, no force transmitting connection between the membrane and the driving/receiving unit, so that the efficiency of the field device is reduced compared with a piezoelectric driving/receiving unit.
Besides the applied driving/receiving unit, diverse electronic components, which usually are integrated into a field device as part an electronics unit, are limiting for the maximum process temperature, at which the particular field device can be applied. In order to decouple such temperature sensitive electronic components from a process, an established method provides integration of a so-called temperature spacing tube in the structure of a field device. For example, involved is a tube, which is part of the housing of the field device, and which is manufactured of a material distinguished by a high thermal insulation. In this regard, reference is made, for example, to EP2520892A1, in which is described the embodying of a section of the housing of a measuring device in such a manner that, in the presence of a temperature difference between an environment of the process connection and the electronics unit, a lower heat flow to the electronics unit occurs in parallel with a longitudinal axis of the housing.
In order to assure an as efficient as possible temperature decoupling of the driving/receiving unit from a process, known from the yet unpublished German patent application No. 102015104536.2 is an apparatus for determining and/or monitoring at least one process variable of a medium in a containment, in the case of which the driving/receiving unit is spatially separated from the process. This German patent application is incorporated by reference in the following.